The overall objective of the proposed research is to comprehend the development of neuronal patterns and differentiation of neuroreceptors in the organ of Corti. The project comprises studies of the development of the isolated organ in culture as compared with that of the intact animal. Of special value is use of the tissue culture technique. We propose the following projects. 1. Ultrastructural study of the developing fetal cochlea in culture. The project is related to development of the innervation pattern in the organ of Corti of the fetal mouse. Three problems will be considered: a) the developmental sequence in the formation of afferent synapses in the fetal organ of Corti, b) the rlation between the differentiation of the hair cell and the acquisition of the synapse, and c) the morphology of the nerve endings of the spiral fibers during the period of innervation-delay of the outer hair cells, a phenomenon especially pronounced in cultures of fetal material. 2. Light-microscope study of the organ of Corti of the Bronx Waltzer mutant mouse in culture. The mutant provides an organ of Corti with a selective population of outer hair cells due to a progressive loss of inner hair cells. The project aims to study the neuronal patterns resulting from the selective loss of inner hair cells. 3. Ultrastructural study of the developing organ of Corti in situ of the Bronx Waltzer mutant mouse to observe the reaction of the afferent synaptic endings and their postsynaptic sites to the degeneration of the receptor cell. 4. Study of the vesiculated nerve endings in culture will be pursued in cultures explanted at different postnatal ages when such endings are known to develop in the intact animal. 5. Study of the patterns of synaptic ribbons in deafferented cultures. Of special interest will be a follow-up of preliminary findings observed in denervated hair cells: a - retaining of synaptic ribbons by the outer hair cells. b - increase in the number of free ribbons. c - formation of reciprocal presynaptic complexes between directly apposed hair cells.